Plasma torches are quite generally used for the thermal machining of electrically conductive materials such as steel and nonferrous metals. In this case, plasma welding torches for welding and plasma cutting torches for cutting electrically conductive materials such as steel and nonferrous metals are used. Plasma torches usually consist of a torch body, an electrode, a nozzle and a holder therefor. Modern plasma torches additionally have a nozzle protective cap fitted over the nozzle. Often, a nozzle is fixed by means of a nozzle cap.
The components that become worn during operation of the plasma torch on account of the high thermal load brought about by the arc are, depending on the plasma torch type, in particular the electrode, the nozzle, the nozzle cap, the nozzle protective cap, the nozzle protective cap holder and the plasma-gas conveying and secondary-gas conveying parts. These components can be easily changed by an operator and thus be referred to as wearing parts.
The plasma torches are connected via lines to a power source and a gas supply which supply the plasma torch. Furthermore, the plasma torch can be connected to a cooling device for a cooling medium, for example a cooling liquid.
Particularly high thermal loads occur in plasma cutting torches. These are caused by the great constriction of the plasma jet by the nozzle bore. Here, by contrast with plasma welding, small bores are used with regard to the cutting current in order that high current densities of 50 to 150 A/mm2 in the nozzle bore, high energy densities of about 2×106 W/cm2 and high temperatures of up to 30 000 K are generated. Furthermore, relatively high gas pressures, generally up to 12 bar, are used in the plasma cutting torch. The combination of high temperature and great kinetic energy of the plasma gas flowing through the nozzle bore result in the workpiece melting and the molten material being driven out. A cutting kerf is produced and the workpiece is separated. In plasma cutting, use is often also made of oxidizing gases in order to cut unalloyed steels. This also additionally leads to a high thermal load on the wearing parts and the plasma cutting torch.
The plasma cutting torch will be addressed in particular below.
A plasma gas flows between the electrode and the nozzle. The plasma gas is conveyed by a gas conveying part, which can also be a multipart part. In this way, the plasma gas can be directed in a targeted manner. Often it is set in rotation about the electrode by a radial and/or axial offset of the openings in the plasma-gas conveying part. The plasma-gas conveying part consists of electrically insulating material since the electrode and the nozzle have to be electrically insulated from one another. This is necessary since the electrode and the nozzle have different electrical potentials during operation of the plasma cutting torch. In order to operate the plasma cutting torch, an arc, which ionizes the plasma gas, is generated between the electrode and the nozzle and/or the workpiece. In order to strike the arc, a high voltage can be applied between the electrode and nozzle, said high voltage ensuring that the section between the electrode and nozzle is pre-ionized and thus an arc is formed. The arc burning between the electrode and nozzle is also referred to as pilot arc.
The pilot arc passes out through the nozzle bore and meets the workpiece and ionizes the section to the workpiece. In this way, the arc can form between the electrode and workpiece. This arc is also referred to as main arc. During the main arc, the pilot arc can be switched off. However, it can also continue to operate. During plasma cutting, it is often switched off in order not to additionally load the nozzle.
In particular the electrode and the nozzle are subjected to high thermal stresses and have to be cooled. At the same time they also have to conduct the electrical current which is required to form the arc. Therefore, materials with good thermal conductivity and good electrical conductivity, generally metals, for example copper, silver, aluminum, tin, zinc, iron or alloys in which at least one of these metals is contained, are used therefor.
The electrode often consists of an electrode holder and an emission insert which is produced from a material which has a high melting point (>2000° C.) and a lower electron work function than the electrode holder. When non-oxidizing plasma gases, for example argon, hydrogen, nitrogen, helium and mixtures thereof, are used, tungsten is used as material for the emission insert, and when oxidizing gases, for example oxygen, air and mixtures thereof, nitrogen/oxygen mixture and mixtures with other gases, are used, hafnium or zirconium are used as materials for the emission insert. The high-temperature material can be fitted into an electrode holder which consists of material with good thermal conductivity and good electrical conductivity, for example pressed in with a form fit and/or force fit.
The electrode and nozzle can be cooled by gas, for example the plasma gas or a secondary gas which flows along the outer side of the nozzle. However, cooling with a liquid, for example water, is more effective. In this case, the electrode and/or the nozzle are often cooled directly with the liquid, i.e. the liquid is in direct contact with the electrode and/or the nozzle. In order to guide the cooling liquid around the nozzle, a nozzle cap is located around the nozzle, the inner face of said nozzle cap forming with the outer face of the nozzle a coolant space in which the coolant flows.
In modern plasma cutting torches, a nozzle protective cap is additionally located additionally outside the nozzle and/or the nozzle cap. The inner face of the nozzle protective cap and the outer face of the nozzle or of the nozzle cap form a space through which a secondary or protective gas flows. The secondary or protective gas passes out of the bore in the nozzle protective cap and encloses the plasma jet and ensures a defined atmosphere around the latter. In addition, the secondary gas protects the nozzle and the nozzle protective cap from arcs which can form between these and the workpiece. These are referred to as double arcs and can result in damage to the nozzle. In particular when piercing the workpiece, the nozzle and the nozzle protective cap are highly stressed by hot material splashing up. The secondary gas, the volumetric flow of which can be increased during piercing compared with the value during cutting, keeps the material splashing up away from the nozzle and the nozzle protective cap and thus protects them from damage.
The nozzle protective cap is likewise subjected to high thermal stress and has to be cooled. Therefore, materials with good thermal conductivity and good electrical conductivity, generally metals, for example copper, silver, aluminum, tin, zinc, iron or alloys in which at least one of these metals is contained, are used therefor.
However, the electrode and the nozzle can also be cooled indirectly. In this case, they are in touching contact with a component which consists of a material with good thermal conductivity and good electrical conductivity, generally a metal, for example copper, silver, aluminum, tin, zinc, iron or alloys in which at least one of these metals is contained. This component is in turn directly cooled, i.e. it is in direct contact with the usually flowing coolant. These components can simultaneously serve as a holder or receptacle for the electrode, the nozzle, the nozzle cap or the nozzle protective cap and dissipate the heat and supply the power.
It is also possible for only the electrode or only the nozzle to be cooled with liquid. It is precisely in this case that excessive temperatures often occur at the only gas-cooled component, which then quickly becomes worn or is even destroyed. This also results in high temperature differences between the components in the plasma cutting torch and as a result in mechanical tensions and additional stresses.
The nozzle protective cap is usually cooled only by the secondary gas. Arrangements in which the nozzle protective cap is cooled directly or indirectly by a cooling liquid are also known.
Gas cooling (plasma-gas and/or secondary-gas cooling) has the drawback that it is not effective for achieving acceptable cooling or dissipation of heat and the required gas volumetric flow is very high for this purpose. Plasma cutting torches with water cooling require for example gas volumetric flows of 500 l/h to 4000 l/h, while plasma cutting torches without water cooling require gas volumetric flows of 5000 to 11 000 l/h. These ranges arise depending on the cutting currents used, which may be for example in a range from 20 to 600 A. At the same time, the volumetric flow of the plasma gas and/or the secondary gas should be selected such that the best cutting results are achieved. Excessive volumetric flows, which are required for cooling, however, often impair the cutting result.
In addition, the high gas consumption brought about by high volumetric flows is uneconomical. This applies particularly when gases other than air, for example argon, nitrogen, hydrogen, oxygen or helium, are used.
The use of direct water cooling for all wearing parts is, by contrast, very effective, but results in an increase in the dimensions of the plasma cutting torch since, for example, cooling channels are required for conveying the cooling liquid to the wearing parts to be cooled and away therefrom again. In addition, when the directly liquid-cooled wearing parts are changed, a great deal of care is necessary since as little cooling liquid as possible should remain between the wearing parts in the plasma cutting torch, since this can result in damage of the plasma torch when the arc is struck.